1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus capable of forming an image on a long sheet and a density correction method used in the image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a uniformly-charged photoconductor (for example, a photoconductor drum) with (to) light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing at a fixing section, whereby an image is formed on the sheet.
An image forming apparatus includes various rotational members such as a photoconductor and a developer bearing member, and is known to cause cyclic density uneveness in the sub scanning direction on an image due to rotational runout of the rotational members. For example, the interval (development gap) between the photoconductor and the developer bearing member is cyclically changed due to rotational runout of the photoconductor or the developer bearing member, and thus the electric field intensity is cyclically changed even when a constant developing bias is applied. As a result, on the image, density uneveness is caused in a cycle same as the rotation cycle of the photoconductor or the developer bearing member.
In a conventional image forming apparatus, correction data corresponding to a rotation position (phase relative to a home position) of a photoconductor is created based on a density profile representing density variation in the sub scanning direction such that the cyclic density uneveness is offset, for example. The correction data is used to correct image formation conditions such as the light exposure energy (light exposure time or light exposure output), the charging voltage, the developing bias voltage, and the rotational frequency of the developer bearing member (for example, developing roller), and the density value (gradation value) of input image data (for example, Japanese Patent Application Laid-Open Nos. 2014-116711 and 2013-195586).
FIG. 1 illustrates a relationship between a density profile and density uneveness on an image when an image is continuously formed on a long sheet. As illustrated in FIG. 1, in the case of an image in which multiple label images are provided in a spaced relationship in a predetermined pattern, the smallest repeating pattern (in FIG. 1, a pattern having five label images) is referred to as an image.
Typically, when an image is continuously formed on a long sheet as illustrated in FIG. 1, the cycle of the density profile and the cycle of image formation differ from each other. That is, the relationship between the density profile and the image is different among the images. Consequently, the correction data used for density correction differs among the images.
In the case where images are formed on flat sheets, a patch image elongated in the sub scanning direction is formed by utilizing the intervals between sheets and the density of the patch image is detected, to periodically acquire the density profile. Therefore, the density correction can be accurately performed by periodically acquiring the density profile. However, when an image is continuously formed on a long sheet, the interval between sheets cannot be ensured, and therefore the density profile cannot be periodically acquired. For this reason, correction data created based on the initial density profile is used to perform density correction; however, when the density profile changes with time, the difference in color appearance is increased among the images, and constant image quality among the images may not be maintained.